Immunotherapy has shown great potential in the treatment of a variety of cancers such as breast cancer, melanoma, renal cell carcinoma and non-Hodgkin's lymphoma; however, the full potential of immunotherapy has not been achieved despite new antigen-specific vaccines, cellular therapy, antibodies, and cytokines. The reasons for the low response rate in cancer immunotherapy are many, but one of the most fundamental issues is that cancer patients develop progressive immunosuppression characterized by decreased T-cell effector and memory function. Experiments in the laboratory of Dr. Andrew Weinberg have described an immunological regulatory molecule present on T cells called OX40. The consequences of OX40 engagement relevant to the proposed research include generation of memory T cells, increased migration of effector T cells into the periphery, enhanced cytokine production by effector T cells, and the ability to break peripheral T-cell tolerance in vivo. OX40+ T cells are present in a wide variety of human tumors and in tumor-involved lymph nodes. OX40-based therapy causes tumor regression in a variety of animal tumor models, and was well-tolerated in murine and non-human primate pre-clinical experiments. This research will be the first to study OX40-based cancer therapy in humans. The specific aims of the proposed research are to define a well-tolerated dose of anti-OX40 for use in future clinical trials in humans by performing a phase I clinical trial, characterize the primary and recall immune responses to reporter antigens after anti-OX40 treatment using established immunological monitoring techniques such as ELISA and cytokine flow cytometry, and to produce a soluble form of the human OX40 ligand that contains the receptor-binding domain and matches or exceeds the biologic activity of the OX40 agonist antibody.